The present invention relates to methods of preparing electrochemical cells.
A polymer electrochemical cell, such as a rechargeable lithium ion battery, commonly constructed by means of the lamination of electrode and separate cell elements which are individually prepared. Each of the electrodes and the electrolyte film/separator is formed, for example, by coating, extrusion, or otherwise, from compositions including binder materials and a plasticizer.
In the construction of a standard lithium-ion cell, for example, an anodic current collector may be positioned adjacent an anode (negative electrode) film, or sandwiched between two separate anode films, to form the negative electrode.
Similarly, a cathodic current collector may be positioned adjacent a cathode (positive electrode) film, or sandwiched between two separate cathode films, to form the positive electrode.
A separator is positioned between the negative electrode and the positive electrode. The anode, separator, and cathode structures are then laminated to produce a unitary flexible cell structure.
An extraction process is used to prepare the cell for activation with electrolyte. During processing of the cell, a large quantity of a homogeneously distributed plasticizer is present in the solid polymeric matrix. Prior to activation of the cell or battery, however, the plasticizer is removed. This is generally accomplished using an extracting solvent such as diethyl ether or hexane, or the application of a vacuum, which selectively extracts the plasticizer without significantly affecting the polymer matrix. This produces a xe2x80x9cdryxe2x80x9d cell, which does not include any electrolyte solvent or salt. An electrolyte solvent and electrolyte salt solution is imbibed into the xe2x80x9cdryxe2x80x9d cell structure to yield a functional electrochemical cell system.
A lithium ion battery typically comprises several cells in which the current from each of the cells is accumulated by a conventional current collector, so that the total current generated by the battery is roughly the sum of the current generated from each of the individual cells in the battery. In lithium ion batteries it is common to stack separate cells to create the battery.
Lithium ion electrochemical cells may be of the traditional xe2x80x9csandwichxe2x80x9d type with a cathode, a separator, and an anode sandwiched together. However, there is a trend to develop xe2x80x9cbi-cellsxe2x80x9d. Bi-cells include a central electrode (either cathode or anode), with the opposing electrode present on each side of the central electrode, separated by membrane or separator-layers.
The activating materials are generally electrolyte solutions added to fill up the porous structure created upon the removal of the plasticizers. Since activation of the cells is typically an expensive and time consuming step, and since a battery""s capacity is dependent on the activation step, there is a need for improvement.
The invention is directed to a method for distributing electrolyte within a laminate structure during activation to form a battery. The laminate structure is placed in a container, then an electrolyte solution comprising an electrolyte solvent and an inorganic salt is placed in the container. In one aspect, compression and decompression cycles are applied to the container in any desired order. The compression and decompression facilitate the distribution of electrolyte over all surfaces of the laminate structure. In one embodiment, the compression cycle is applied followed by decompression. In another embodiment, the decompression cycle is applied followed by compression.
A further feature of the invention is to apply the compression cycle and the decompression cycle more than once. This provides for improvement of wetting the laminate structures over single compression and decompression cycles.
A further feature of the invention is to use flexible containers for holding the laminate structures during activation. The flexible containers allow for the transmission of external forces into the laminate structure.
In one embodiment of the invention, the decompression cycle is accomplished by subjecting the container to an external vacuum, the compression cycle is accomplished by releasing the vacuum. Preferably the steps are repeated.
In another embodiment of the invention, the compression cycle is accomplished by subjecting the container to an external pressure, the decompression cycle is accomplished by releasing the pressure. Preferably the steps are repeated.
In another embodiment of the invention, the compression cycle is accomplished by mechanically applying a force to squeeze the container, the decompression cycle is accomplished by releasing the mechanically applied force Preferably the steps are repeated.
Another embodiment is the use of ultrasound for the compression and decompression cycles. The laminate structures are placed in a container and electrolyte solution is added. The container is then subjected to ultrasonic waves.